This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the presently described embodiments. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present embodiments. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Blowout preventers (“BOPs”) are used extensively throughout the oil and gas industry. Typical BOPs are used as a large specialized valve or similar mechanical device that seal, control, and monitor oil and gas wells. The two categories of BOPs that are most prevalent are ram BOPs and annular BOPs. BOP stacks frequently utilize both types of BOPs, typically with at least one annular BOP stacked above several ram BOPs. The ram units in ram BOPs allow for shearing drill pipe in the case of shear rams, sealing off around drill pipe in the case of pipe rams, and sealing the BOP bore in the case of blind rams. Typically, a BOP stack may be secured to a wellhead and may provide a safe means for sealing the well in the event of a system failure.
An example BOP includes a main body or housing with a vertical bore. Ram bonnet assemblies may be bolted to opposing sides of the main body using a number of high tensile fasteners, such as bolts or studs. These fasteners are required to hold the bonnet in position to enable the sealing arrangements to work effectively. An elastomeric sealing element may be used between the ram bonnet and the main body. There are several configurations, but essentially they are all directed to preventing a leakage bypass between the mating faces of the ram bonnet and the main body.
Each bonnet assembly includes a piston which is laterally movable within a ram cavity of the bonnet assembly by pressurized hydraulic fluid acting on the piston. The opposite side of each piston has a connecting rod attached thereto which in turn has a ram mounted thereon for extension into the vertical bore. The rams can be shear rams for shearing an object within the bore of a BOP. Alternatively, the rams can be pipe rams for sealing off around an object within the bore of a BOP, such as a pipe, thereby sealing the annular space between the object and the BOP bore. In addition, the rams can be blind rams for sealing the BOP bore.
The rams are designed to move laterally toward the vertical bore of the BOP to shear or seal off on any object located therein. For instance, opposing shear rams utilize cutting surfaces configured to close in on and shear an object located in the BOP bore, such as a section of drill pipe used during drilling operations. The opposing shear rams can include seals disposed adjacent the cutting faces of the shear rams and configured to come together to seal off the BOP bore. Placement of the seals near the cutting faces of shear rams weakens the cutting or shearing strength of the shear rams.
Pipe rams utilize seals that close in on and seal off on a tubular within the vertical bore of the BOP. Each pipe ram typically has a semicircular opening in its front face to form a seal about half of the outer periphery of the object within the BOP vertical bore. When the opposing pipe rams are closed, the opposing pipe rams engage each other and seal the entire periphery of the object, thereby closing off the annulus between the object and the BOP bore. Typical pipe ram assemblies can include a ram packer which is composed of an elastomeric or rubber material configured to seal off against the tubular within the vertical bore of the BOP when the opposing rams are run into the closed position.
Still further, blind rams utilize seals that close in and seal on opposing blind ram seals to seal off a BOP bore when no object is present in the bore.
As discussed above, a single BOP stack can include an annular BOP, one or more sets of shear rams, one or more sets of pipe rams, and one or more sets of blind rams. Each respective set of rams is located above or below each adjacent set of rams. In this way, a BOP stack increases in height and weight with the addition of each set of rams. It is desirable to minimize the height and weight of a BOP stack to reduce complexity, costs, and equipment footprint.
Further, many existing BOP rams are constructed of a material which, while suitable for cutting or shearing purposes, is not NACE International compliant. That is, the rams and in particular the sealing features of the rams do not meet the requisite corrosion control standards.
Moreover, and as discussed above, placement of seals or packers adjacent the cutting edge of a shear ram can affect the shear ram cutting or shearing strength.
Accordingly, shear, blind, and pipe rams including sealing features that are NACE compliant are desirable. Furthermore, shear, blind, and pipe rams which do not unnecessarily add to the height and weight of a BOP stack are desirable. Still further, a shear ram with sealing capability but with uncompromised cutting edges is desirable. In addition, a subsea drilling system capable of being pressure tested without having to pull the entire drilling riser is desirable.